The present invention relates to an auto focus mechanism in an image input apparatus. And more particularly, it relates to an auto focus mechanism capable of obtaining a clear image by adjusting a focal distance of an optical system in accordance with each part thereof in a direction of installing the optical system, even if a manuscript mounted on a manuscript board of an input apparatus is not evenly in contact with an image input plane, that is, even if a focal distance of the manuscript is different at each part thereof.
Usually, a focal distance of an image read optical unit of a scanner is adjusted so as to be adapted to a target of reading (manuscript) fittingly mounted on an image input plane of a manuscript board. If the target of reading is a sheet of manuscript, the focal distance can be adjusted with respect to the manuscript since the entire manuscript is adhered to the image input plane of the manuscript board. However, if the manuscript is, e.g., an opened book or the like, a part of the target of reading corresponding to a bind-in portion of the opened book is distanced from the image input plane, so that the focal distance can not be adjusted to that portion.
Further, as means for adjusting the focal distance, if a conventional auto focus mechanism using a movement adjustment mechanism for moving a position of, e.g., a CCD sensor or a movement adjustment mechanism or the like for moving a position of a SELFOC lens (GRIN—gradient index—lens) is applied, a manuscript such as the above opened book can be theoretically read by automatic focus adjustment. Such a conventional auto focus mechanism is provided to a read mechanism comprising a long optical system which is parallel with one side of the manuscript board under the manuscript board made of, e.g., a rectangular glass and a moving mechanism for reciprocating this optical system in one direction along the other side of the manuscript board.
The conventional auto focus mechanism automatically adjusts the focal position of the optical system by the above structure. Therefore, if the target of reading whose focal distance differs at each part of a manuscript such as the opened book has the same focal distance along the direction of the installation of the optical system (one side of the manuscript board), i.e., if the bind-in portion of the book is mounted along one side of the manuscript board, the part of the manuscript lifted up from the image input plane can be adjusted by the same automatic focus adjustment operation, and the part of the manuscript lifted up from the glass plate as well as the part of the manuscript adhered to the glass plane of the mounting board can be also read as a substantially good image.
However, even the image input apparatus having such an auto focus mechanism can not accurately adjust the focal position of the manuscript which is, for example, obliquely lifted up from the optical system for reading, and an excellent input image can not be obtained.